Semiconductor devices are formed on a substrate of high-purity single-crystal semiconductor, which is fabricated by, for example, pulling a cylindrical single-crystal from the melt in a crucible, as well-known as the Czochralski method hereinafter called the CZ method. In the CZ method, polysilicon material is charged in the crucible inside a single-crystal semiconductor pulling apparatus. The polysilicon material is heated and melted by means of heaters around the crucible. A seed held by a seed holder is immersed in the melt, and then is pulled while rotating the seed holder and the crucible in the same direction or the opposite directions to grow the single-crystal semiconductor.
As compared with single-crystal grown along the &lt;100&gt;, &lt;111&gt; or &lt;511&gt; orientation that has been mass-produced by using the CZ method, with a single-crystal grown along the &lt;110&gt; orientation it is more difficult to remove the dislocation in a so-called diameter-reducing step in which dislocation in a crystal is removed by growing the crystal from the seed. The reason is that, in the single-crystal grown along the &lt;110&gt; orientation, the dislocation is located in the orientation parallel to the axis of the seed crystal, i.e., in the vertical direction in the single-crystal semiconductor pull apparatus, and the same as the growth direction of the single-crystal, the dislocation extends along the &lt;110&gt; orientation. As a solution of this, a so-called multi-step diameter-reducing method has been employed by which a diameter-reduced portion is made to have large or small diameters in a range of 3-6 mm. Referring to FIG. 2 showing a schematic view of the diameter-reduced portion to which the diameter-reducing method has been performed, the diameter of a diameter-reduced portion 2 is gradually made smaller from the point where it is connected to the seed 1 and thereafter the diameter is enlarged to have a larger diameter d1 of 4-6 mm and the is reduced to have a smaller diameter d2 of 3-4 mm. The dislocation is removed by repeating this step three times or more. A shoulder fabrication step is then carried out.
However, the diameter-reducing step carried out in the general hot zone without a magnetic field is affected by the heat convection of the silicon melt, inert gas blowing, crucible rotation, and so on. Therefore, the melt surface is vibrated and when crystal diameter is reduced to be in the range of 2.0-3.5 mm, the crystal at the melt surface is apt to crack. If the diameter is increased to prevent cracking, the dislocation cannot be removed. Therefore, it has been considered that the single-crystal semiconductor grown along the &lt;110&gt; orientation cannot be mass-produced by the pulling method.
Two issues in the fabrication process of the &lt;110&gt; single-crystal are quite important for putting it in mass-production. The premier issue is to prevent the crystal from cracking in the diameter-reducing step, and the second, is to minimize the increase in cost in connection with the cracking prevention.